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BIOCHEMICAL COMPOSITION AND FUNCTIONS OF BLOOD

AND LYMPH

BLOOD:

In humans, blood is an opaque red fluid, freely flowing but denser and more viscous

than water. The characteristic colour is imparted by hemoglobin, a unique iron-

containing protein. Hemoglobin brightens in colour when saturated with oxygen

(oxyhemoglobin) and darkens when oxygen is removed (deoxyhemoglobin). For this

reason, the partially deoxygenated blood from a vein is darker than oxygenated blood

from an artery. The red blood cells (erythrocytes) constitute about 45 percent of the

volume of the blood, and the remaining cells (white blood cells, or leukocytes, and

platelets, or thrombocytes) less than 1 percent. The fluid portion, plasma, is a clear,

slightly sticky, yellowish liquid. After a fatty meal, plasma transiently appears turbid.

Within the body the blood is permanently fluid, and turbulent flow assures that cells and

plasma are fairly homogeneously mixed.

The total amount of blood in humans varies with age, sex, weight, body type, and other

factors, but a rough average figure for adults is about 60 millilitres per kilogram of body

weight. An average young male has a plasma volume of about 35 millilitres and a red

cell volume of about 30 millilitres per kilogram of body weight.

BLOOD FACTS

Approximately 8% of an adult’s body weight is made up of blood.

Females have around 4-5 litres, while males have around 5-6 litres. This

difference is mainly due to the differences in body size between men and

women.

Its mean temperature is 38 degrees Celcius.

It has a pH of 7.35-7.45, making it slightly basic (less than 7 is considered

acidic).

Whole blood is about 4.5-5.5 times as viscous as water, indicating that it is more

resistant to flow than water. This viscosity is vital to the function of blood because

if blood flows too easily or with too much resistance, it can strain the heart and

lead to severe cardiovascular problems.

Blood in the arteries is a brighter red than blood in the veins because of the

higher levels of oxygen found in the arteries.

An artificial substitute for human blood has not been found.

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FUNCTIONS OF BLOOD

Blood has three main functions: Transport, Protection and Regulation.

TRANSPORT:

Blood transports the following substances:

Gases, namely oxygen (O2) and carbon dioxide (CO2), between the lungs and rest of

the body

Nutrients from the digestive tract and storage sites to the rest of the body

Waste products to be detoxified or removed by the liver and kidneys

Hormones from the glands in which they are produced to their target cells

Heat to the skin so as to help regulate body temperature

PROTECTION:

Blood has several roles in inflammation:

Leukocytes, or white blood cells, destroy invading microorganisms and cancer cells

Antibodies and other proteins destroy pathogenic substances

Platelet factors initiate blood clotting and help minimise blood loss

REGULATION:

Blood helps regulate:

pH by interacting with acids and bases

Water balance by transferring water to and from tissues

COMPOSITION OF BLOOD

Blood is classified as a connective tissue and consists of two main components:

1. Plasma, which is a clear extracellular fluid

2. Formed elements, which are made up of the blood cells and platelets

The formed elements are so named because they are enclosed in a plasma membrane

and have a definite structure and shape. All formed elements are cells except for the

platelets, which are tiny fragments of bone marrow cells.

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Formed elements are:

Erythrocytes, also known as red blood cells (RBCs)

Leukocytes, also known as white blood cells (WBCs)

Platelets

Leukocytes are further classified into two subcategories called granulocytes which

consist of neutrophils, eosinophils and basophils; and agranulocytes which consist of

lymphocytes and monocytes.

The formed elements can be separated from plasma by centrifuge, where a blood

sample is spun for a few minutes in a tube to separate its components according to their

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densities. RBCs are denser than plasma, and so become packed into the bottom of the

tube to make up 45% of total volume. This volume is known as the haematocrit. WBCs

and platelets form a narrow cream-coloured coat known as the buffy coat immediately

above the RBCs. Finally, the plasma makes up the top of the tube, which is a pale

yellow colour and contains just under 55% of the total volume.

BLOOD PLASMA

Blood plasma is a mixture of proteins, enzymes, nutrients, wastes, hormones and

gases. The specific composition and function of its components are as follows:

PROTEINS

These are the most abundant substance in plasma by weight and play a part in a variety

of roles including clotting, defence and transport. Collectively, they serve several

functions:

They are an important reserve supply of amino acids for cell nutrition. Cells called

macrophages in the liver, gut, spleen, lungs and lymphatic tissue can break down

plasma proteins so as to release their amino acids. These amino acids are used by

other cells to synthesise new products.

Plasma proteins also serve as carriers for other molecules. Many types of small

molecules bind to specific plasma proteins and are transported from the organs that

absorb these proteins to other tissues for utilisation. The proteins also help to keep

the blood slightly basic at a stable pH. They do this by functioning as weak bases

themselves to bind excess H+ ions. By doing so, they remove excess H+ from the

blood which keeps it slightly basic.

The plasma proteins interact in specific ways to cause the blood to coagulate, which

is part of the body’s response to injury to the blood vessels (also known as vascular

injury), and helps protect against the loss of blood and invasion by foreign

microorganisms and viruses.

Plasma proteins govern the distribution of water between the blood and tissue fluid

by producing what is known as a colloid osmotic pressure.

There are three major categories of plasma proteins, and each individual type of

proteins has its own specific properties and functions in addition to their overall

collective role:

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1. Albumins, which are the smallest and most abundant plasma proteins. Reductions in

plasma albumin content can result in a loss of fluid from the blood and a gain of fluid

in the interstitial space (space within the tissue), which may occur in nutritional, liver

and kidney disease. Albumin also helps many substances dissolve in the plasma by

binding to them, hence playing an important role in plasma transport of substances

such as drugs, hormones and fatty acids.

2. Globulins, which can be subdivided into three classes from smallest to largest in

molecular weight into alpha, beta and gamma globulins. The globulins include high

density lipoproteins (HDL), an alpha-1 globulin, and low density lipoproteins (LDL), a

beta-1 globulin. HDL functions in lipid transport carrying fats to cells for use in energy

metabolism, membrane reconstruction and hormone function. HDLs also appear to

prevent cholesterol from invading and settling in the walls of arteries. LDL carries

cholesterol and fats to tissues for use in manufacturing steroid hormones and

building cell membranes, but it also favours the deposition of cholesterol in arterial

walls and thus appears to play a role in disease of the blood vessels and heart. HDL

and LDL therefore play important parts in the regulation of cholesterol and hence

have a large impact on cardiovascular disease.

3. Fibrinogen, which is a soluble precursor of a sticky protein called fibrin, which forms

the framework of blood clot. Fibrin plays a key role in coagulation of blood, which is

discussed later in this article under Platelets.

AMINO ACIDS:

These are formed from the break down of tissue proteins or from the digestion of

digested proteins.

NITROGENOUS WASTE:

Being toxic end products of the break down of substances in the body, these are usually

cleared from the bloodstream and are excreted by the kidneys at a rate that balances

their production.

NUTRIENTS:

Those absorbed by the digestive tract are transported in the blood plasma. These

include glucose, amino acids, fats, cholesterol, phospholipids, vitamins and minerals.

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GASES:

Some oxygen and carbon dioxide are transported by plasma. Plasma also contains a

substantial amount of dissolved nitrogen.

ELECTROLYTES:

The most abundant of these are sodium ions, which account for more of the blood’s

osmolarity than any other solute.

RED BLOOD CELLS

Red blood cells (RBCs), also known as erythrocytes, have two main functions:

1. To pick up oxygen from the lungs and deliver it to tissues elsewhere

2. To pick up carbon dioxide from other tissues and unload it in the lungs

An erythrocyte is a disc-shaped cell with a thick rim and a thin sunken centre. The

plasma membrane of a mature RBC has glycoproteins and glycolipids that determine a

person’s blood type. On its inner surface are two proteins called spectrin and actin that

give the membrane resilience and durability. This allows the RBCs to stretch, bend and

fold as they squeeze through small blood vessels, and to spring back to their original

shape as they pass through larger vessels.

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RBCs are incapable of aerobic respiration, preventing them from consuming the oxygen

they transport because they lose nearly all their inner cellular components during

maturation. The inner cellular components lost include their mitochondria, which

normally provide energy to a cell, and their nucleus, which contains the genetic material

of the cell and enable it to repair itself. The lack of a nucleus means that RBCs are

unable to repair themselves. However, the resulting biconcave shape is that the cell has

a greater ratio of surface area to volume, enabling O2 and CO2 to diffuse quickly to and

from Hb.

The cytoplasm of a RBC consists mainly of a 33% solution of haemoglobin (Hb), which

gives RBCs their red colour. Haemoglobin carries most of the oxygen and some of the

carbon dioxide transported by the blood.

Circulating erythrocytes live for about 120 days. As a RBC ages, its membrane grows

increasingly fragile. Without key organelles such as a nucleus or ribosomes, RBCs

cannot repair themselves. Many RBCs die in the spleen, where they become trapped in

narrow channels, broken up and destroyed. Haemolysis refers to the rupture of RBCs,

where haemoglobin is released leaving empty plasma membranes which are easily

digested by cells known as macrophages in the liver and spleen. The Hb is then further

broken down into its different components and either recycled in the body for further use

or disposed of.

WHITE BLOOD CELLS

White blood cells (WBCs) are also known as leukocytes. They can be divided into

granulocytes and agranulocytes. The former have cytoplasms that contain organelles

that appear as coloured granules through light microscopy, hence their name.

Granulocytes consist of neutrophils, eosinophils and basophils. In contrast,

agranulocytes do not contain granules. They consist of lymphocytes and monocytes.

GRANULOCYTES:

1. Neutrophils: These contain very fine cytoplasmic granules that can be seen under a

light microscope. Neutrophils are also called polymorphonuclear (PMN) because they

have a variety of nuclear shapes. They play roles in the destruction of bacteria and

the release of chemicals that kill or inhibit the growth of bacteria.

2. Eosinophils: These have large granules and a prominent nucleus that is divided into

two lobes. They function in the destruction of allergens and inflammatory chemicals,

and release enzymes that disable parasites.

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3. Basophils: They have a pale nucleus that is usually hidden by granules. They secrete

histamine which increases tissue blood flow via dilating the blood vessels, and also

secrete heparin which is an anticoagulant that promotes mobility of other WBCs by

preventing clotting.

AGRANULOCYTES:

1. Lymphocytes: These are usually classified as small, medium or large. Medium and

large lymphocytes are generally seen mainly in fibrous connective tissue and only

occasionally in the circulation bloodstream. Lymphocytes function in destroying

cancer cells, cells infected by viruses, and foreign invading cells. In addition, they

present antigens to activate other cells of the immune system. They also coordinate

the actions of other immune cells, secrete antibodies and serve in immune memory.

2. Monocytes: They are the largest of the formed elements. Their cytoplasm tends to be

abundant and relatively clear. They function in differentiating into macrophages,

which are large phagocytic cells, and digest pathogens, dead neutrophils, and the

debris of dead cells. Like lymphocytes, they also present antigens to activate other

immune cells.

PLATELETS

Platelets are small fragments of bone marrow cells and are therefore not really

classified as cells themselves.

Platelets have the following functions:

1. Secrete vasoconstrictors which constrict blood vessels, causing vascular spasms in

broken blood vessels

2. Form temporary platelet plugs to stop bleeding

3. Secrete procoagulants (clotting factors) to promote blood clotting

4. Dissolve blood clots when they are no longer needed

5. Digest and destroy bacteria

6. Secrete chemicals that attract neutrophils and monocytes to sites of inflammation

7. Secrete growth factors to maintain the linings of blood vessels

The first three functions listed above refer to important haemostatic mechanisms in

which platelets play a role in during bleeding: vascular spasms, platelet plug formation

and blood clotting (coagulation).

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LYMPH

Lymph is the fluid that circulates throughout the lymphatic system. The lymph is formed

when the interstitial fluid (the fluid which lies in the interstices of all body tissues)[1] is

collected through lymph capillaries. It is then transported through lymph vessels to

lymph nodes before emptying ultimately into the right or the left subclavian vein, where

it mixes back with blood.

Since the lymph is derived from the interstitial fluid, its composition continually changes

as the blood and the surrounding cells continually exchange substances with the

interstitial fluid. Lymph returns proteins and excess interstitial fluid to the bloodstream.

Lymph may pick up bacteria and bring them to lymph nodes where they are destroyed.

Metastatic cancer cells can also be transported via lymph. Lymph also transports fats

from the digestive system.

COMPOSITION OF LYMPH

1)Water: 94%,

2)Solids: 6%. The solid content found in the lymph are as follows:

i)Proteins: Total protein content varies from-2.0-4.5%. Three varieties of proteins are

found–albumin, globulin and fibrinogen. In addition to this, traces of prothrombin are

also found. Fibrinogen content is very low.

ii)Fats: In fasting condition fat content is low but after a fatty diet it may be 5.0-15%.

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iii)Carbohydrates: Sugar, 132.2 mg per 100 ml.

iv)Other Constituents: Expressed in mg per 100ml. urea, 23.5 mg; non-protein

nitrogenous substance, 34.8 mg; creatinine, 1.4 mg; chlorides, 711 mg; total

phosphorus, 118 mg; inorganic phosphorus, 5.9 mg; calcium, 9.84 mg. Enzymes and

antibodies are also present.

Lymph has a composition comparable to that of blood plasma, but it may differ slightly.

Lymph contains white blood cells. In particular the lymph that leaves a lymph node is

richer in lymphocytes. Likewise, the lymph formed in the digestive system called chyle

is rich in triglycerides (fat), and looks milky white.

Lymph contains a variety of substances, including proteins, salts, glucose, fats, water,

and white blood cells. Unlike your blood, lymph does not normally contain any red

blood cells.

The composition of lymph varies a great deal, depending on where in your body it

originated. In the lymphatic vessels of your arms and legs, lymph is clear and

transparent, and its chemical composition is similar to blood plasma (the liquid portion

of blood). However, lymph contains less protein than plasma.

The lymph returning from your intestines is milky, owing to the presence of fatty acids

absorbed from your diet. This mixture of fats and lymph is called chyle, and the special

lymphatic vessels surrounding your intestine that collect chyle are called lacteals.

Lacteals drain into a dilated sac - the cisterna chyli (meaning reservoir for chyle) - at

the lower end of the thoracic duct. (The cisterna chyli can be seen in the above

diagram of the lymphatic ducts.) The thoracic duct then conveys the chyle to your

bloodstream, where the fats it carries can be processed for energy or storage.

FUNCTIONS OF LYMPH

The various functions of lymph include:

1)Nutritive: It supplies nutrition and oxygen to those parts where blood cannot reach.

2)Drainage: It drains away excess tissue fluid and the metabolites and in this way tries

to maintain the volume and composition of tissue fluid constant.

3)Transmission of Proteins: Lymph returns proteins to the blood from the tissue

spaces.

4)Absorption of Fats: Fats from the intestine are also absorbed through the

lymphatics.

5)Defensive: The lymphocytes and monocytes of lymph act as defensive cells of the

body. The lymph also removes bacteria from tissues.

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BIBLIOGRAPHY

http://www.britannica.com/EBchecked/topic/69685/blood/257799/Blood-components

http://www.myvmc.com/anatomy/blood-function-and-composition/

http://en.wikipedia.org/wiki/Lymph

http://education-portal.com/academy/lesson/what-is-a-lymph-definition-anatomy-

quiz.html#lesson

http://www.indiastudychannel.com/resources/136108-Composition-functions-

Lymph.aspx